In recent years and continuing, demand for innovating optical transmission systems is growing along with increase in transmission traffic. The same degree of transmission distance and frequency usability as those in the conventional 10 Gbit/s systems is required for the next-generation optical transmission systems. To achieve this, practical use of digital coherent optical communication schemes, which are superior in optical signal-to-noise ratio (OSNR) tolerance and nonlinearity tolerance compared to non-return-to-zero (NRZ) modulation scheme of the conventional systems, has been promoted.
FIG. 1 illustrates a conventional digital coherent optical transceiver 100. The optical transceiver 100 uses separate light sources for the transceiver and the receiver. A laser diode (LD) module 106 is used for transmission and an LD module 104 is used as a local oscillator source on the receiver side. A digital signal processor (DSP) 102 applies a prescribed modulation scheme to inputted data signals to cause a driver 105 to drive a modulator 107. The modulator 107 modulates continuous light emitted from the LD module 106 by data driving signals and outputs the modulated signals from the optical transmitter 100.
On the receiving side, an optical signal received at a receiver 103 is subjected to separation of polarized components, and each component interferes with the corresponding component of local oscillator light emitted from the local oscillator LD module 104 to extract an in-phase component and an orthogonal component. The DSP 102 carries out synchronization between the received signal and the local oscillator light, mitigates linear distortion due to wavelength dispersion, and demodulates the received signal as an electric signal.
In each of the LD modules 104 and 106, a high reflective coating is provided to the rear end face and an anti-reflection coating is provided to the output face (or the front end face) of the laser device.
A technique for monitoring an output from the rear end face of a laser device is proposed to maintain the light level emitted from the front end face of the laser device constant. See, for example, Japanese Laid-open Patent Publication No. 2000-124541. Another technique for superimposing light beams output from the front end face and the rear end face of a laser diode on an object to be measured is proposed to reduce a size and power consumption of a speed meter. See, for example, Japanese Laid-open Patent Publication No. 2005-140619. Still another technique for arranging micro prisms to the front end face and the rear end face of each of the laser devices of a laser array to deflect the light beams output from the front end face and the rear end face is also proposed. See, for example, Japanese Laid-open Patent Publication No. 2009-135312. With this technique, the irradiation angle of the light is widened.
The layout illustrated in FIG. 1 in which a transmitted light source and a receiving light source are separately used is disadvantageous from the viewpoint of the device size and power consumption.
It is desired to use a single light source module, while controlling transmitted light and local oscillator light independently from each other, to make an optical transceiver compact and reduce power consumption.